1. Field of the Invention
The present invention relates to a magnetic core type laminated inductor. More specifically, the present invention is effective for application to a surface mounting chip inductor used in the state of direct-current superposition, and is suitable for application to a micro DC-DC converter in a mobile information device such as a mobile telephone, which is configured to convert a power supply voltage (an electromotive force) obtained from an internal battery into a given circuit operating voltage.
2. Description of the Related Art
Magnetic core type inductors such as transformers or choke coils used in power circuits including DC-DC converters and the like are formed by winding coils around magnetic cores. Therefore, it has been difficult to achieve downsizing, or more particularly, to achieve thinner profiles of the inductors as compared to electronic components such as semiconductor integrated circuits. Accordingly, the inventors of the present invention have studied a magnetic core type laminated inductor as shown in FIGS. 9A to 9D.
FIGS. 9A to 9D show a configuration of a magnetic core type laminated inductor for which the inventors have studied prior to the present invention. Of these drawings, FIG. 9A is a perspective view of an external configuration, FIG. 9B is a top plan view of conductive patterns, FIG. 9C is a cross-sectional view taken along A-A line in FIG. 9B, and FIG. 9D is an enlarged view in a thickness direction of FIG. 9C, respectively.
A non-magnetic core type laminated inductor, which has no magnetic core, is formed by laminating a non-magnetic electrical insulating layer and conductive patterns by screen printing or the like, whereas a magnetic core type laminated inductor 10b shown in FIGS. 9A to 9D is formed by laminating electrical insulating magnetic bodies (soft magnetic bodies) 30 and conductive patterns 20 by screen printing or the like. The conductive patterns 20 overlap in the direction of the layer in the electrical insulating magnetic bodies 30, thereby forming a coil L that extends spirally. The laminated electrical insulating magnetic bodies 30 form a closed magnetic circuit that guides magnetic fluxes (shown by the arrows in the drawing) from the coil L annularly. Both ends of the coil L are connected to electrode terminals 11 and 12 located on both ends of an inductor chip through lead conductive pattern portions 21 and 22.
The magnetic core type laminated inductor 10b includes the magnetic core made of the magnetic bodies 30, and is therefore capable of reducing magnetic leakage and obtaining necessary inductance with relatively a small number of turns of the coil. For this reason, this configuration is suitable for forming the above-mentioned transformer or choke coil into a micro chip inductor. For example, in terms of a chip inductor used for a high frequency switching DC-DC converter, the configuration can deal with almost any specification requirements with about 4 turns of the coil in combination with the magnetic bodies 30 having high magnetic permeability.
Here, other publicly known technical examples relatively close to the studied technique include laminated inductors disclosed in Japanese Patent Application Laid-open Publications Nos. 2003-31424 and 2001-85231, for instance.
The magnetic core type laminated inductor 10b can obtain high inductance as compared to the number of turns of the coil. However, the inductor has a problem that the inductance rapidly drops even at a small coil current (an exciting current) due to magnetic saturation of the magnetic bodies 30. In other words, the inductor has a problem that it is not possible to achieve a sufficient rated current as a transformer or a choke coil because of a small current upper limit that can assure the inductance equal to or above a given level.
An inductor applied to a supply circuit or a power circuit such as a DC-DC converter is often used in the state of direct-current superposition, i.e. while superposing direct currents. It is necessary to ensure the rated current to a sufficiently large level in order to obtain a given inductance characteristic in the state of direct-current superposition.
Therefore, the inventors have studied a technique to enhance a magnetic saturation level of the closed magnetic circuit by interposing a magnetic gap layer 40 in the closed magnetic circuit as shown in FIGS. 10A and 10B, and thereby to increase the rated current.
Of FIGS. 10A and 10B, FIG. 10A shows a cross-sectional view enlarged in the thickness of the magnetic core type laminated inductor 10b and FIG. 10B shows a current/inductance characteristic graph of the inductor 10b, respectively.
As shown in FIG. 10A, the magnetic core type laminated inductor 10b illustrated in the drawing includes four layers (20a to 20d) of conductive patterns 20 formed in the magnetic bodies 30 having high magnetic permeability. The four-layered conductive patterns (20a to 20d) form a coil having four turns. The magnetic gap layer 40 is formed in a central layer portion so as to bisect the four-layered conductive patterns (20a to 20d) in the direction of the layers. Since this magnetic gap layer 40 is interposed in the closed magnetic circuit, it is possible to enhance the magnetic saturation level in the closed magnetic circuit.
In this way, as shown in FIG. 10B, it is possible to ensure a high current upper limit, i.e. a large rated current which can assure an inductance value equal to or above a given level. In the graph shown in FIG. 10B, a solid line shows a characteristic when the magnetic gap layer 40 is present, and a dashed line shows a characteristic when the magnetic gap layer 40 is absent.
The magnetic core type laminated inductor 10b shown in FIG. 10A can increase the rated current so as to assure the inductance value equal to or above the given level by use of the magnetic gap layer 40. However, the following problems are found out.
Specifically, in terms of FIG. 10B, variation in inductance attributable to the coil current is relatively gentle in a region where the coil current (the exciting current) is larger than a certain level. However, the inductance is distinctively high in a region where the coil current is small, and the variation attributable to the coil current is steep and the characteristic is not stable. Accordingly, in the case of using the inductor while superposing direct currents, the inductor poses a problem that the superimposed current suffers significant fluctuation of the inductance and a favorable performance of direct-current superposition can be therefore obtained.
Meanwhile, it is usually effective to carry out measurement and inspection of the inductance at a small current in light of reduction in a burden of measurement and enhancement in inspection efficiency. However, the inspection at a small current measures the distinctively high inductance as well. Accordingly, there is also a problem of incapability to carry out correct inspection.
To the knowledge of the inventors, the following is a conceivable reason of the distinctively high inductance at the small current region. Specifically, locally closed magnetic circuits are formed around the respective conductive patterns (20a to 20d) as indicated with arrows in FIG. 10A. Due to interposition of the magnetic gap layer 40, the closed magnetic circuits having relatively low magnetic permeability are locally formed around the inner conductive patterns 20b and 20c adjacent to the magnetic gap layer 40. Meanwhile, due to absence of interposition of the magnetic gap layer 40, the closed magnetic circuits having relatively high magnetic permeability are locally formed around the outer conductive patterns 20a and 20d distant from the magnetic gap layer 40. For this reason, induced magnetic fluxes from the respective conductive patterns are not mutually balanced and cancelled between the inner conductive pattern 20b or 20c and the outer conductive pattern 20a or 20d, and a local magnetic bias is thereby generated. It is conceivable that local magnetic saturation generated by this magnetic bias causes the distinctively high inductance as shown in FIG. 10B.